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Unless what you meant is that if today there is nothing like bitcoin anymore. Then I look for a good altcoin and continue. Or do you mean the crash of cryptocurrency as a whole. That wouldn't be possible of course, unless you have to shutdown the internet.
If any critical problem will destroy bitcoin, then no altcoin can stand on its place, as for at the moment. Bitcoin still is perceived like all crypto for too many, so its safety impacts all crypto. Maybe someday some other projects can become enough independent, but that das hasn't come yet. That's why it is so important for bitcoin devs to be very attentive and careful and even simetimes conservative. Bitcoin's success is important for all of us.